The invention relates to a liquid dosing device, comprising an open-ended vitreous capillary tube which is connected to a reservoir for liquid, heating means for heating the liquid in the reservoir to a given temperature T.sub.1, and a ceramic actuator element which is in intimate contact with a portion of the external surface of the tube, the tube and the element being mutually adhered using an adhesive substance. Examples of liquids which can be dosed with such a device include molten solder, glue and flux.
Such a device is known from U.S. Pat. No. 4,828,886, in which the liquid is molten solder. A device of this type is commonly employed in so-called "solder jet" machines, which are used to aim and fire small balls of solder at given surfaces, e.g. those of printed wiring boards. To this end, molten solder in the vitreous capillary tube is ejected therefrom with the aid of the ceramic actuator (e.g. a piezoelectric or electrostrictive actuator), which can be used to apply small pressure pulses to the tube in response to an appropriate electrical input signal. Such pressure pulses cause a slight, temporary contraction of the tube wall, which leads to the forceful expulsion of a droplet of solder from the open end of the tube (remote from the reservoir). Solder expelled from the tube in this manner is continually replenished with fresh solder from the reservoir, under pressure.
Since the amplitude of the actuator's mechanical displacement is relatively small, any mechanical play between the actuator and the tube could cause a significant decrease in the efficiency of the liquid expulsion process; consequently, the tube and actuator element are preferably mutually adhered, so as to ensure good mechanical contact. In practice, this is achieved by providing a layer of polymeric adhesive (such as an epoxy polymer) between the tube and the actuator.
A disadvantage of the known device is that, as time progresses, the reliability of the liquid expulsion process often tends to deteriorate, leading to substantial variations in the drop size or drop range of the expelled liquid.